Building a garden patch

I'm planning to build a garden patch at my concrete floor backyard. How do I build the raised bed in a way that allows water to drain and garantee enough soil for vegetables to grow. I was thinking of using treated wood to build it.But how high should it be? Should I cover it with an impermeable plastic layer?
Thank you for your help
Ruth (Portugal)

Answers

I'm not too sure of the answer to this one so I'm going to direct you to another member - Ickledigga. She has built some amazing raised beds which look like a good height and sufficient enough to grow root vegetables. The only thing I would suggest is that you drill holes along the bottom plank of the structure to allow water to drain out.. You could line the edges with plastic. Make sure you fill with topsoil, grit and compost. Put an inch or 2 of rubble on the bottom of the structure before you pop in the soil.

Hi Ruth, welcome to GOY.
As you have a concrete base, you will have to think about where the water goes from your raised bed. I would suggest you buy a deep metal cold frame with which you can control the watering.
You can have more control over your crops with a cold frame in your climate.
Waste soiled water from a raised bed will block your drains, better controlled from a cold frame.

With no 'access' to the natural subsoil, I think I would be most concerned that the soil would rather quickly become stale due to lack of natural movement of moisture in the soil, which brings minerals up from the subsoil that the veggies need - you will need to feed your soil maybe more than you would need to if the plants were in the ground.. It's just a thought.......

Many people with little or no access to soil grow container veg all the time, there are bags you can buy for spuds, I grew tomatoes in pots last year, buy a couple of barrels and an arch and grow pumpkins over the arch. Onions and garlic do great in pots, as do carrots. You are limited with some veg, but there are lots of options. I would try and find a book that concentrates on container veg gardens. I found this on amazon.com

Thank you all for your suggestions. I had a glimpse at the book and it does look fantastic. I will most surely buy it.
I have more or less an ideia of how to start building it now. And I'm hoping to start planting by beginning of February.
Regarding that matter, your advice would be to grow seeds indoors and then transplant them or buy tiny plants to plant them directly in my vegetable patch?

Regards seeds versus plants, I only have a little plot - about 3m x 3m - and I find that it is just as well to buy plants in the case of 'big' plants like cauliflowers, as I can't fit many in and sowing seeds in trays can be risking, esp early in the season. Not sure what you'll be growing in Portugal tho......really depends on that.